Adobe Ink is a stylus and Adobe Slide is a ruler. Both of them are priced at $199 and individual gadget can't be bought.
I tried Adobe Ink and Slide with Adobe Line app. I was super impressed with the fact that I could easily draw straight line, circles with smooth edges, rectangles, different shaped triangles etc. It was super easy to draw most of the geometrical shapes on iPAD. And it's not only about geometrical shapes, there are many intelligent features offered by these gadgets which help in connecting different elements to make something interesting. These gadgets can be very useful for creative professionals and folks who work on drawings. E.g. - Architects.
Adobe shares more about Ink and Slide at - http://www.adobe.com/in/products/ink-and-slide.html
"Adobe introduced this creative hardware that makes sketching and drawing more natural and fluid. These are designed to be used with the iPad, Adobe Ink is a fine-tip, pressure-sensitive pen built using Adonit Pixelpoint technology to give you greater creative control and unprecedented precision. Adobe Slide is a digital ruler that lets you draw any shape on your iPad — including straight lines, perfect circles, classic French curves, and more."
Here is another interesting article about Adobe Ink and Slide - http://www.forbes.com/sites/anthonykosner/2014/06/18/adobe-launches-ink-the-cloud-pen-to-reimagine-drawing-in-the-digital-workflow/